This Invention relates to a laser device for oscillating or amplifying a laser beam by supplying an excitation beam to a laser active material contained in a long-sized laser beam guide portion, such as an optical fiber, a method of producing the same, and a composite optical medium for use in producing the same and, in particular, to a laser device, a method, and a composite optical medium of the type which are effectively applicable to the fields of optical communication, optical measurement, and laser machining.
In the fields of optical communication, optical measurement, and laser machining, it is desired to develop a laser device which is higher in output power or efficiency but is lower in cost. Presently, a fiber laser device is known as one of candidates which may possibly satisfy the above-mentioned demand.
The fiber laser device comprises an optical fiber as a laser beam guide portion (a so-called laser medium). The optical fiber comprises a core portion containing a laser active material and a cladding portion coaxially surrounding the core portion. By appropriately selecting the diameter of the core portion and the difference in optical refractive index between the core portion and the cladding portion, a single transverse mode of laser oscillation is relatively easily achieved.
In addition, by confining a light beam within the optical fiber at a high density, it is possible to enhance the interaction between the laser active material and the light beam. Since the length of interaction can be prolonged by increasing the length of the optical fiber, it is possible to generate a laser beam having a high quality in spatial characteristics at a high efficiency. Thus, the laser beam of an excellent quality can be obtained at a relatively low cost.
In order to achieve a higher output power and a higher efficiency of the laser beam, it is required to efficiently introduce an excitation beam to a laser active region (typically, the core portion) of the optical fiber so that the excitation beam is sufficiently absorbed in the laser active material, such as laser active ions, pigments, or any other emission center, added to the laser active region.
However, in order to satisfy a single-mode waveguide condition, the diameter of the core portion must be smaller than 20 xcexcm. Generally, it is difficult to efficiently introduce the excitation beam to the core portion having such a small diameter.
In order to overcome the above-mentioned difficulty, proposal is made of a fiber laser device and a laser machining device in Japanese Unexamined Patent Publication (JP-A) No. H11-284255. The laser device comprises a laser fiber which has a core containing a laser active material and which is for producing a laser beam from its output end when the laser active material is excited. The laser fiber is, directly or indirectly through an optical medium, brought into contact with an optical guide structure adapted to confine an excitation beam for exciting the laser active material. The laser active material is excited by the excitation beam incident through a contact portion between the laser fiber and the optical guide structure.
Thus, the optical guide structure adapted to confine the excitation beam is used as an excitation beam guide portion. Through the excitation beam guide portion, the excitation beam is introduced from a side surface of the laser fiber as a laser beam guide portion. The excitation beam is introduced in a distributed state in which it is distributed along the length of the laser beam guide portion. By the excitation beam introduced into the laser beam guide portion in the distributed state, the laser active material contained in the laser beam guide portion is excited.
In this case, the introduction of the excitation beam into the excitation beam guide portion can be carried out through an incident prism portion formed at a desired position of the excitation beam guide portion. The excitation beam incident to the excitation beam guide portion is repeatedly reflected in the interior of the excitation beam guide portion to spread throughout the interior of the excitation beam guide portion. Then, the excitation beam is introduced into the laser beam guide portion through the contact portion where the excitation beam guide portion is directly or indirectly brought into contact with the side surface of the laser beam guide portion.
For example the excitation beam guide portion comprises a hollow cylindrical structure or a flat disk-shaped structure. The excitation beam guide portion is supplied with the excitation beam from an excitation light source and transmits the excitation beam which is confined therein by internal reflection. The laser beam guide portion is brought into optical contact with the surface of the excitation beam guide portion over a predetermined length. For example, the laser beam guide portion in the form of a fiber is wound around the structure forming the excitation beam guide portion. Thus, the excitation beam confined within the excitation beam guide portion is incident into the laser beam guide portion through the contact portion.
With the laser device having the above-mentioned excitation structure, it is easy to introduce the excitation beam into the laser beam guide portion in the form of a fiber. In addition, the excitation beam can be introduced through a desired position on the structure forming the excitation beam guide portion. This facilitates the excitation by the use of a plurality of excitation light sources. In contrast, the excitation beam is introduced only through opposite end faces of the fiber in the conventional laser device.
The conventional laser device described above is produced through the steps of forming the structure as the excitation beam guide portion and winding the laser beam guide portion in the form of a fiber around the structure. However, this process involves the following problem.
In the above-mentioned conventional laser device, optical coupling between the excitation beam guide portion and the laser beam guide portion is weak or at least insufficient. In order to make the laser beam guide portion fully absorb the excitation beam, it is required to extend the area of distributed introduction of the excitation beam. For this purpose, the laser beam guide portion must be brought into contact with the excitation beam guide portion over a distance as long as possible.
In other words, in order to efficiently excite the laser active material contained in the laser beam guide portion, the laser beam guide portion must be sufficiently long. Consequently, the excitation beam guide portion must have a surface area sufficient to receive the laser beam guide portion of such a long size attached or wound therearound. This is an inhibiting factor which makes it difficult to improve the efficiency of the laser device of the type and to reduce the cost.
As described above, the excitation beam guide portion must have a large surface area in order to extend the area of distributed introduction of the excitation beam into the laser beam guide portion. On the other hand, the excitation beam guide portion is required to have an internal volume as small as possible in order to reduce transmission loss of the excitation beam. If the internal volume of the excitation beam guide portion is large, the transmission distance of the excitation beam within the excitation beam guide portion is increased to thereby increase the transmission loss.
For example, the excitation beam guide portion is formed into a hollow cylindrical shape or a flat disk shape. In order to reduce the transmission loss, the structure must be as thin as possible. However, when such a thin structure is formed by molding or machining an optical material such as a glass, the production cost inevitably becomes high.
Furthermore, the above-mentioned problems will cause another problem of decreasing the degree of freedom related to the shape of the laser device.
As described above, in the laser device comprising the long-sized flexible laser beam guide portion such as the optical fiber and the excitation beam guide portion for introducing the excitation beam into the laser beam guide portion in the distributed state, optical coupling between the excitation beam guide portion and the laser beam guide portion is weak. In order to enhance the excitation efficiency by overcoming the weakness of the optical coupling, the production cost inevitably becomes high because of the problem in the production process and the degree of freedom related to the shape of the laser device is decreased.
It is therefore an object of this invention to provide a laser device comprising a laser beam guide portion containing a laser active material and an excitation beam guide portion for guiding an excitation beam, which is capable of assuring sufficient optical coupling between the laser beam guide portion and the excitation beam guide portion to efficiently introduce the excitation beam into the laser beam guide portion and which is capable of achieving a high degree of freedom related to the shape of the laser device.
It is another object of this invention to provide a method of producing the laser device mentioned above.
It is still another object of this invention to provide a composite optical medium adapted to form the laser device mentioned above.
According to a first aspect of this invention, there is provided a laser device comprising a longitudinal flexible composite optical medium (that is, a long-sized flexible composite optical medium) which comprises:
a laser beam guide portion containing a laser active material; and
an excitation beam guide portion for distributing and introducing an excitation beam into the laser beam guide portion;
the laser beam guide portion and the excitation beam guide portion being uncoaxially arranged in parallel to each other and integrally coupled with each other, the composite optical medium being wound or folded to form a predetermined shape.
Preferably, at least one part of the excitation beam guide portion of the composite optical medium is optically connected to another part of the excitation beam guide portion which is adjacent to the one part as a result of winding or folding.
Preferably, at least one part of the excitation beam guide portion of the composite optical medium is integrally coupled by the use of fusion-bonding, optical contact, or resin to another part of the excitation beam guide portion which is adjacent to the one part as a result of winding or folding.
Preferably, at least one part of the excitation beam guide portion of the composite optical medium is integrally coupled, to form an integrally coupled portion, by the use of fusion-bonding, optical contact, or resin with another part of the excitation beam guide portion which is adjacent to the one part as a result of winding or folding, the laser device further comprising an excitation beam introducing portion formed at the integrally coupled portion for introducing the excitation beam into the excitation beam guide portion.
Preferably, the laser device further comprises a plurality of excitation beam introducing portions formed at a plurality of positions of the excitation beam guide portion of the composite optical medium wound or folded, the excitation beam introducing portions being for introducing the excitation beam into the excitation beam guide portion.
According to a second aspect of this invention, there is provided a method of producing a laser device comprising a laser beam guide portion containing a laser active material and an excitation beam guide portion for distributing and introducing an excitation beam into the laser beam guide portion, the method comprising the steps of:
forming a longitudinal flexible composite optical medium comprising the laser beam and the excitation beam guide portions, each of which has a long size and a flexibility and which are uncoaxially arranged in parallel to each other and integrally coupled with each other; and
winding or folding the composite optical medium to form a structure of a predetermined shape.
Preferably, the forming step is for forming the longitudinal flexible composite optical medium by drawing or wiredrawing preforms of the laser beam and the excitation beam guide portions which are brought into tight contact or integrally coupled with each other.
Preferably, the winding or folding step is for winding or folding the composite optical medium to form the structure of the predetermined shape so that turned or folded parts of the excitation beam guide portion of the composite optical medium are stacked in a layered fashion.
According to a third aspect of this invention, there is provided a composite optical medium comprising:
a laser guide portion; and
an excitation beam guide portion;
the laser beam guide portion comprising:
a core portion containing a laser active material; and
a cladding portion coaxially surrounding the core portion;
the excitation beam guide portion being uncoaxially arranged in parallel to the laser beam guide portion and forming distributed optical coupling with the cladding portion, the composite optical medium having a long total length as compared with its sectional area to thereby be given a flexibility allowing the composite optical medium to be wound or folded.
Preferably, the laser beam guide portion and the excitation beam guide portion are integrally coupled with each other so as to extend in parallel to each other and to be aligned with each other in a transversal direction.
According to the above-mentioned aspects of this invention, the excitation beam can efficiently be introduced into the laser beam guide portion with sufficient optical coupling assured between the laser beam guide portion containing the laser active material and the excitation beam guide portion for guiding the excitation beam. Furthermore, the laser device with a high degree of freedom related to the shape can be obtained. In addition, such laser device can simply be formed.